As described in Wolfe's U.S. Pat. No. 5,190,879 (filed 1991), millions of laboratory animals have been used every year in experimental research. These animals range from mice to non-human primates. In order to conduct valid and reliable experiments, researchers must be assured that their animals are protected from pathogens and microbial contaminants that could affect test results and conclusions.
There are presently at least 1300 research facilities and 223 federal agencies registered with the U.S. Department of Agriculture (USDA) that use registered laboratory animals. (Crawford, "A review of the animal welfare enforcement report data, 1973-1995," AWIC Newsletter, Summer 1996) These facilities include institutions, organizations and corporations such as hospitals, colleges and universities, diagnostic and toxicology laboratories, pharmaceutical companies and biotechnology companies. In 1995, these combined organizations used a total of 1,395,463 registered animals, of which 333,379 were Guinea pigs and 248,402 were hamsters. In addition to these registered facilities, there are nearly a thousand Institutional Animal Care and use Committees (IACUC) that report to the Public Health Service (PHS), mostly the National Institutes of Health. Some 90 percent of the animals used at these institutions are mice and rats, but exact figures for mice and rats are not known, since they are not registered animals. Nevertheless, it is estimated that the United States uses between 15.3 and 18.7 million mice and rats a year. (See Mukerjee, "Trends in Animal Research," Scientific American, February 1997, pp. 86-93 and Stephens, "A Current View of Vivisection Animal Research in America," The Animal's Agenda, September/October 1996, pp. 20-25.) In Canada, 1.25 million mice and 650,000 rats were reported (by Mukergee, supra) to have been used in 1992 for research. These figures exclude permanent breeding populations at research institutions and commercial suppliers, which are estimated to number 1.5 million mice and 500,000 rats.
Many laboratory animals in the past have suffered subclinical infections, in which they did not demonstrate any overt signs of disease. Because more research is now being conducted at the molecular and microscopic level, these subclinical infections are being discovered and are invalidating research. Various studies have demonstrated that contamination and compromised animal integrity are pervasive problems in the United States. The loss of biological integrity results in significant losses in valuable research time and money in laboratory animal research.
Since the conditions of housing and husbandry affect animal and occupational health and safety as well as data variability, and influence an animal's well-being, the present invention relates to a biological barrier/isolator caging system for laboratory animals to permit optimum environmental conditions and animal comfort. Because of risks of contamination, biocontainment requirements, DNA hazardous issues, gene transfer technologies disease induction, allergen exposure in the workplace and animal welfare issues, current caging system technologies would appear insufficient to support the modern biotechnology industry. The objective of the invention is to attain performance standards such as the exclusion of pathogenic and opportunistic organisms, containment of biological products, hazardous materials, allergens and bioaerosols, elimination of intracage contaminants and control and maintenance of an optimal microenvironment. The invention should also have the effect of improving laboratory animal housing conditions.
The health quality of research animals has recently improved enormously, creating a need for specialized caging equipment. Animal suppliers around the world have experienced an unprecedented demand for defined pathogen-free animals, and are now committed to the production and accessibility of such animals to researchers. The needs for improvement and technological advancement for efficiently, safely and comfortably housing laboratory animals arise mainly from contemporary interests in pathogen-free, immunocompromised, immunodeficient, transgenic and induced mutant ("knockout") animals. Transgenic technologies, which are rapidly expending, provide most of the animal populations for modeling molecular biology applications. Transgenic animals account for the continuous success of modeling mice and rats for human diseases, models of disease treatment and prevention and by advances in knowledge concerning developmental genetics. Also, the development of new immunodeficient models has seen tremendous advances in recent years due to the creation of gene targeted models using knockout technology.
The number of publications on these subjects has increased from 64 for transgenic and one for knockout in 1986 to 1726 and 496, respectively, in 1996, based upon a Medline search. Further projections through Medline search trends predict that about 2454 papers will be published in the year 2001. Estimating the numbers of animals required at 200 per report, this means an estimated 500,000 mice will need to be maintained under proper barrier caging. The pharmaceutical industry presents new trends in research, resulting in a marked shift from acute to chronic disease studies. They incorporate the technology of genetic engineering into the traditional medicinal chemistry research process.
Unfortunately, transgenic animals are very often contaminated or "dirty" animals, because of the lack of proper animal care facility resources to protect the health of the newly-created animals. The current scientific advances and opportunities raise complex questions that must be addressed by researchers and animal care professionals. These questions include how to manage risks to compromised animals, to research personnel and the society at large of animal-to-human disease transmission through genetic manipulation, and how and whether to provide adequate resources for research and breeding applications of the new mutants. It has been suggested that animals expressing pathogenic transgenes may suffer from unique diseases. In light of the risks of transmission of disease to the animal users, some mechanism is needed to ensure attention to adequate biocontainment and health protection of transgenics and knockout mutants for minimizing exposure and for continued contamination control. Also, it is essential to consider that infectious agents, opportunistic organisms, allergens, airborne contaminants, fomites and environmental factor fluctuations have the potential to induce animal stress and diseases and variability in research or testing data. Animals become more vulnerable to diseases and more susceptible to human and cross-contamination as we use immunocompromised and genetically altered mutants.
Scientists refuse to use animals that are not healthy and cared for properly. Illness, undue stress or poor living conditions would interfere with obtaining valid, useful results from scientific experiments using animals. In brief, excellent science requires excellent care. The value of the animals used in biomedical research has increased substantially with the advent of gene transfer technology. For instance, the cost of a single transgenic white mouse could easily exceed $100,000 when the time and effort required to effect a successful gene transfer is considered. (Cooper, "Design Considerations for Research Animal Facilities," Lab Animal, September 1989, pp. 23-26.) These lines of animals are not only extremely valuable but also frequently irreplaceable. Therefore, they need to be provided with the highest quality environments and protected from cross-contamination. The living conditions for such animals must be kept at or near their ideal environment. Therefore, barriers at cage level must be provided to ensure both exclusion and containment in environments appropriate for the species. Transgenic technology will certainly become more important in the future, and with the contemporary world harmonization of animal welfare standards, it is necessary to ensure that the animal's (product) investment is protected. The caging systems of the present invention will satisfy scientific expectations.
These new laboratory technologies require a larger number of animal cages to be maintained in the same floor space. The present invention provides means of reducing facility construction costs and animal husbandry-related expenses without jeopardizing the quality of the care provided to the animals or the value of the scientific research conducted. Transgenic colony management is very expensive, especially at a time when animal rights activism is increasing research animal care costs. As animal purchase and maintenance costs steadily increase while grant funds decrease, cost containment of the transgenic colonies used in research becomes increasingly important. Since labor is the greatest single cost, reducing labor is the key to reducing overall costs. Since specialized microisolation cages and labor costs are both significant, substantial reduction of caging costs will help to accomodate research users. For example, as with long-term testing and the associated risk of losing a colony, research managers are required to decide whether to use mass air systems and whether they need clean rooms, with their high installation costs, to provide high quality animals and avoid possible losses of time and data. Installing clean rooms can require expenditures of $400 to $500 per square foot, so the availability of the caging systems of the present invention will reduce these costs by offering protection similar to a mass air room, but at cage level. The present invention also provides a type of automation for changing cages, to eliminate the cost of bedding and bedding-related activities including bedding ordering, receiving, storage, dispensing, autoclaving, dust removal, bedding disposal, cage-scraping, bagging, disposal and removal of soiled bedding.
Many animal pathogens can become airborne or travel on fomites such as dust. Therefore, open-system caging operations present a risk of contamination. Most research institutions are presently caging mice in filter-top cages (at a cost of about $65 per cage plus $140 for ancillary equipment), which have been shown to reduce concentrations of airborne pathogens as well as allergens, compared with conventional open-top cages (which cost about $40 per cage plus $30 for ancillary equipment). Rodent cages with filter-tops create a contaminant barrier at the cage level. However, they restrict ventilation, prevent heat dissipation and affect the quality of animal research data. Ventilated (positive pressure, open system) cage and rack systems (which cost about $130 per cage plus $150 for ancillary equipment) that protect animal health and reduce exposure to airborne contaminants are commercially available. However, they are expensive and leak pathogens into the workers' environment. Cage and rack systems that exhaust air through a HEPA filter system before returning it into the room substantially reduce the concentration of airborne allergens, but are very expensive (about $200 per cage plus $150 for ancillary equipment). Such cage and rack systems are used in barrier animal facilities, which cost around $400-600 per square foot. Besides increasing the cost of housing mice significantly, such systems tend to invalidate research data and adversely affect animal well-being. They are still open systems which leak into the work environment, thus exposing both animals and workers to potential risks of contamination and allergies. In proportional terms, caging mice in filter-top cages costs approximately 293 percent more than caging them in open cages. Caging in HEPA ventilated cage and rack systems costs about 400 percent more, while caging in HEPA-ventilated in-and-out cage and rack systems would cost about 500 percent more.
In addition to protecting animals from extraneous cross-contamination, there is a need to isolate laboratory personnel from allergens that are indigenous to a species or hazardous agents that are experiment specific. For example, many technicians and scientists are troubled by allergic reactions to animal dander. Allergens are also found in the urine of mice and rats. There is also the threat of contracting contagious diseases that are present in animal studies. Animals may become contaminated at the research facility or in transit, where they are exposed to the outside environment.
As reported in the Denver Post, Jan. 1, 1998, two researchers at the Yerkes Regional Primate Research Center of Emory University have recently been exposed to the hepatitis B virus via contact with caged research monkeys, and the first worker died Dec. 10, 1997. In an era when research animals are infected with various virulent diseases, clearly it would be desirable to provide improved protection to research staff, which can be accomplished by the caging systems of the present invention.
The current technology (as described in U.S. Pat. No. 5,190,879) for isolating small laboratory animals in research facilities includes filtered air hoods, filtered air housing units and filtered air rooms. These systems are very expensive and are stationary in nature. There is currently a trend towards the use of micro-isolation cages, in which only the food, water and bedding have to be changed in a horizontal flow Class 100 air displacement bench or the like. The isolator caging system uses a standard solid bottom (shoebox) cage equipped with a filter top. The top consists of a polycarbonate frame fitted with a piece of filter media. It is made of a spunbonded polyester material known as Reemay.TM. filters that have different particle arrest capabilities. The fabric's ability to pass air is inversely proportional to its particle arrest capability.
Cage manufacturers use different types of Reemay filters, but the most current are the 2024, 2033 and 2295. The Reemay 2024 has an 85 percent atmospheric dust removal efficiency for particles in the 1 to 5 micron range but only a 28 percent efficiency for particles in the 0.3 to 1 micron range. The rim at the bottom of the filter top, where it fits over the underlying cage, is made of a lip, forming a junction design similar to that in a Petri dish. (Lipman, "Microenvironmental conditions in Isolator Cages, An Important Research Variable," Lab Animal, June, 1992, pp. 23-26.) Despite the large exposed filter surface area and the permeability of the filter media, studies have shown that the air exchange in isolator cages does not take place through the filter, but at the junction of the lid with the cage. Additionally, the results showed that the filter top reduced air exchange rates within the cages to less than one air change per hour (ACH) regardless of the ACH rate provided in the animal room. (Keller et al., "Evaluation of Intracage Ventilation in Three Animal Caging Systems," Lab Animal, Vol 39, pp. 237-242, 1989) These caging systems clearly impede intracage ventilation and can lead to an unhealthy microenvironment.
Also available are filter tops that are constructed of pressed pulp. The pressed pulp forms a dense mat of wood fibers that acts as a depth filter to block the passage of microbial contaminants.
There are a variety of gaseous and particulate contaminants that accumulate in the animal's environment. The sources of this pollution include thermal loads generated by metabolic activity, moisture generated from respiration, excrement, and the water source, ammonia generated by bacteria from the breakdown of urea found in excrement, and carbon dioxide generated as a metabolic waste product. These pollutants all need to be removed or diluted via ventilation or else there is a significantly poor microenvironmental air quality. As the magnitude of the differences between isolator cage macro- and microenvironmental conditions became apparent, cage manufacturers developed caging systems that are supply-coupled or directly ventilated, as opposed to the room-coupled or passively ventilated systems described above. (Lipman, supra, 1992)
The use of rodent caging systems that provide individual ventilated isolator cages is rapidly increasing. These systems have been shown to considerably improve the microenvironmental conditions to which rodents are exposed. Ventilated caging systems have also been shown to enhance containment capability at cage level, reducing the opportunity for cross-contamination. In general, these systems provide filtered air directly into the cage, thereby pressurizing it. The positive pressure differentials increase the amount of allergens released into the atmosphere, which may increase the risk of allergies developing in research or animal care personnel. Caging systems may be purchased with exhaust systems that scavenge air as it exits from the junction of the cage top and bottom and/or the cage top filter. Because of a junction design similar to that of a Petri dish, no ventilated system is capable of scavenging all the air escaping from the cage. (Tu et al., "Determination of Air Distribution, Exchange, Velocity and Leakage in Three Individually-Ventilated Rodent Caging Systems," Contemporary Topics, Vol. 36, pp. 69-79, 1997) Air leakage and release of intracage air into the room is an important source of airborne contamination.
The use of laboratory animals in research is increasing rapidly, putting research and testing institutions at ever-increasing risks of occupational health litigation. Employee health problems and occupational hazards caused by animal allergens have become a significant concern at many research facilities. Laboratory animal allergy (LAA) is an important occupational disease that affects between 15 and 44 percent of workers in animal care facilities. (Eggleston, "Death by Dander: Laboratory Animal Allergies in the Workplace," PRIM & R Meeting, San Diego, Calif., Mar. 17, 1997; Olfert, "Allergies to Laboratory Animals--Aspects of Monitoring and Control," Lab Animal, February 1993, pp. 32-35) Currently, it is reported that fifty percent of animal-exposed laboratory research personnel exhibit allergies to the laboratory animals, and three fourths of all institutions with laboratory animals now have animal-care workers with allergic symptoms. For example, a recent study performed at the Karolininska Institute in Sweden revealed that nearly 50 percent of animal-exposed personnel evidenced allergies to lab animals. Up to 73 percent of persons, including scientists and animal-care personnel, with pre-existing allergic conditions such as allergic rhinitis (hay fever) eventually develop allergies to laboratory animals. Ten percent of these persons develop occupation-related asthma. Currently in the U.S., approximately 35,000 workers and 500,000 scientists have been exposed to laboratory animal allergens. These people could be eligible for medical and indemnity compensation. Workers' compensation claims related to animal allergies are estimated at about $50 million for the past three years. Despite these statistics, there appears to be no effort to develop closed-system (leak free) caging and work area technology.
Personnel who are exposed to animal allergens react in such ways as allergic dermatitis, respiratory allergic diseases and anaphylactic syndrome. The asthmatic reactions that are associated with the illness may be life threatening, and chronic occupational asthma can be associated with irreversible lung disease. This is an IgE mediated immune response to allergenic proteins that are produced by the animals and become airborne on small respirable particles. Exposure to laboratory animal fur or dander, saliva, serum or other body tissues should be minimized. This is a legitimate biological concern, and yet there are no technological alternatives. Animal caging with special air filtration for intake and exhaust, contamination-free environments and good air quality systems are the most efficient methods of containing such respirable particles. The annual costs of LAA illnesses could be enormous, including both medical and disability costs and lost productivity. By eliminating allergen exposures in the workplace, it would be possible to improve worker health, maintain animal health and reduce operating costs. The caging systems of the present invention are expected to meet institutional expectations.
The lack of control of environmental conditions such as temperature, relative humidity, ventilation rate and illumination at the animal cage level prevents proper validation of research and testing data and adversely affects animal well-being. The chilling and dehydration of rodent neonates, hairless and nude strains in mechanically ventilated caging systems have caused animal losses due to hypothermia. Thus, there is a growing need for improved caging systems which would safeguard the health of both the laboratory animals and their keepers. Furthermore, keeping social animals such as rodents permanently in barren cages is unacceptable for ethical, professional and scientific reasons. It deters the animals from expressing their normal behaviors and favors stereotypic behaviors instead. It is thus desirable to provide environmentally enhanced caging systems to improve the well-being of laboratory animals and the quality of research conducted on them.
Cages currently used to isolate rodents resemble a Petri dish and have filter tops. They are known as microbarrier (Allentown Caging Equipment, Inc., Allentown, N.J.) or microisolator (Lab Products, Inc., Seaford, Del.) cages. These cages have a proven isolation capability, but restrict ventilation to less than one air change per hour, thus providing poor air quality. These cages still leak airborne contaminants and allergens into the occupational environment. Regardless of the number of air changes per hour in the room, such cages operate the same because of the cage top design. Cage ventilation in filter-top cages is driven by thermal and moisture diffusion through the filter top and by convection across the cage. There is no other escape for thermal currents created by the animals in the cage. Computational fluid dynamics has been used to study filter-top cages in a six-shelf rack with seven cages per shelf. The thermal currents and cumulative effect of metabolic heat load, moisture and toxic gases across the rack were evaluated for each of the 42 cages for five racks. An animal room was set at 66 deg. F., 50% relative humidity and 15 air changes per hour with a changing station in it. It was discovered that there is a lack of consistency from capabilities. With 5-mice residing in a static microisolator cage, the air velocity is 0.05 cfm, providing 0.02 air change per hour with 4.degree. C. temperature rise (Riskowski et al., 1996). This static filter-top cage has a filter membrane, making the airflow independent of room ventilation. The filter top restricts convection and diffusion thermodynamics with the resultant accumulations of temperature, humidity, ammonia, and carbon dioxide over time. The filter-top cage thus provides isolation but lacks microenvironmental comfort, containment, and enrichment capabilities (Keller et al., 1989, (Maghirang, R. G., 1995, Memarzadeh, F., 1998, Riskowski et al., 1996, Reeb, C. K. et al., 1997, Serrano, L. J., 1971). Ventilated filter-top cages provide 40 to 100 ACH depending on manufacturers. Pressurization of the cage by "high-efficiency particulate air" (HEPA) filter/blower supply and/or exhaust modules is independent of room ventilation. Velocities up to 100 fpm (air at 20.degree. C. and moving at 60 linear fpm has a cooling effect approximating 7.degree. C.) in the cage have been recorded, thus inducing cold stress. The individually ventilated filter-top cage provides isolation but lacks microenvironmental comfort, containment and enrichment capabilities (Huerkamp, M. J. and Lehner, D. M., 1994, Lipman, N. S. et al., 1993, Novak, G., 1997, Tu, H. et al., 1997).
Since all three types of caging systems are independent of room ventilation settings, air handling systems are designed to condition macroenvironment or the human occupied zone only. Air handling or HVAC (heating, ventilation, air, conditioning) systems are the most costly component of any animal facility, often consuming 40 to 50 percent or more of the construction budget (Hughes and Reynolds, 1995). Applicant has considered means to create a cost-effective caging system that could be room-coupled, function in a position to position of cages on the same rack and across the room, depending upon where air diffusers and exhausts are located. Nevertheless, depending upon the lower or higher locations of the cages on the rack, microenvironmental variations were noted on the order of 3 deg. F., 10% in relative humidity, 2.256 ppm carbon dioxide, 4.8 ppm ammonia and two times less air changes/hour. The heat stratification from accumulated hot and humid air creates a barrier inside the tops of the cages and under each shelf, affecting the thermal currents. Even though sufficient chilled air is supplied to the room, the chilled air cannot penetrate the barrier of hot air trapped within the cages and under the shelves.
The caging system is an important factor in the physical environment of laboratory animals, the microenvironment. Microenvironmental conditions lack similarity to animal room conditions, the macroenvironment. Animal Welfare regulations (AWA, Guide) prescribe room (but not cage) temperature, humidity and ventilation settings as well as solid bottom cages for microenvironmental animal comfort. There are three types of solid-bottom caging systems currently used in animal facilities. Two types of shoeboxes are room-coupled in a static mode: cages with open tops and cages with filter tops also called microisolator cages. The third type is a shoebox with filter top individually coupled to blower supply and/or exhaust modules and distribution plenums on a rack. Solid bottom cages with open tops provide 10 to 16 air changes per hour (ACH), regardless of the room ventilation (Reeb, C. K. et al., 1997). The thermodynamics of convection and diffusion from the thermal updraft by the heat load of the mice create the airflow. The open top cage provides microenvironmental comfort but lacks isolation, containment, and enrichment static mode, and be coupled to the building HVAC exhaust system.
Computational fluid dynamics (CFD) is a software analysis tool. It uses equations of the conservation of mass, momentum, and energy, which essentially say, "what goes in must come out" (Hughes and Reynolds, 1995). This application describes CFD output used to facilitate the design process and predict air movement in a new type of caging system. Contours, vectors, and particle tracks are examined to adjust microenvironmental comfort. The goal was to meet or exceed all current guidelines and regulations at the cage level, while innovating in cost-effective and appropriate caging systems.
Application of CFD has demonstrated that vented filter-cages with closed-tops can provide 6 to 30 ACH depending on the chosen filter materials and the room ventilation settings. The thermodynamics of convection, buoyant flow and conservation of mass from the thermal updraft resulting from the heat load of the mice create the airflow that is dependent upon room ventilation. This vented closed-system for mice caging is believed to be the only one that will provide microenvironmental comfort, isolation, containment, and enrichment. Applicant conducted a qualitative and quantitative analysis of air distribution pattern, velocity, air change per hour, and leakage in these cages. Also studied were temperature, thermal loads from metabolic activity; humidity, moisture from respiration, wastes, and water source; ammonia, from bacterial breakdown of urea in excrement; and carbon dioxide, as a metabolic waste product, all were monitored over a two week period.
Standardized testing methods for characterizing the design and operation of ventilated caging systems were used by Tu et al. (1997) to define and quantify differences in air distribution, exchange, velocity, and leakage in three commercially available systems. These methods are also published as the National Sanitation Foundation Standard 49 for Class II (Laminar Flow) Biohazard Cabinetry (Ann Arbor, Mich., 1992). The concept is particularly relevant since the driving force for the flow is a result of buoyancy due to temperature gradients. In this case, five mice generate heat loads of 2.0265 Kcal/hr and moisture of 2.5 g of water/hr. Use of laminar convection flow to ventilate the microenvironment through filter membrane would eliminate gaseous buildup and provide good air quality within the enclosure. Therefore, natural convection airflow should provide an efficient room-coupled `closed-system` method for producing adequate microenvironmental ventilation and efficient microbiological barrier at cage level, i.e. provide product and personnel protection.
Subsequent to the filing of the above-identified provisional application, a literature search was performed concerning mice caging, particularly relating to room air distribution and the relationship between macro- and microenvironments, effects of ambient temperature on growth, and moisture production of mice. The following brief summaries discuss pertinent publications.
The Guide for the Care and Use of Laboratory Animals, Institute of Laboratory Animal Resources (1996), National Research Council, National Academy Press, pp. 23-55, discusses macro- and microenviroments for lab animals, including temperature and humidity conditions and the determination of optimal ventilation rates.
Reeb et al. in "Impact of Room Ventilation Rates on Mouse Cage Ventilation and Microenvironment," Contemp. Topics Lab. Anim. Sci., Vol., 36, pp. 74-79 (1997) present a study of non-pressurized, bonnet-topped mouse cages housing four mice each. The effects of room ventilation rate on various aspects of the microenvironments in the cages were examined, and it was found that increasing the room ventilation rate beyond 5 ACH did not result in significant improvements in the cage microenvironments.
Maghirang et al. in "Development of Ventilation Rates and Design Information for Laboratory Animal Facilities," Part I--Field Study, ASHRAE Transactions, Vol. 101, Pt. 2, RP-730 (1995) discussed a survey of animal facilities and their characteristics. It was found that cage conditions varied widely among cages within the same room and among similar cages in different rooms; Cage type was the most important factor that influenced cage conditions and uniformity in cage conditions; and room air exchange rate, air velocity approaching the cage, number of returns and diffusers, and diffuser type did not significantly influence cage conditions and uniformity in cage conditions.
Riskowski et al. in "Development of Ventilation Rates and Design Information for Laboratory Animals Facilities," part 2--Laboratory Tests, ASHRAE Transactions, Vol. 102, Pt. 2, RP-730 discussed the results of tests of conditions in animal rooms and within animal cages at selected locations in the rooms. Conclusions included: Cage conditions varied widely with cage location in a room; Cage type was the most important factor that influenced cage conditions; and Room ACH values from 5 to 15 had the same effects on cage conditions, so the higher room air exchange rates did not provide better conditions for the animals.
Perkins et al. reported in "Characterization and Quantification of Microenvironmental Contaminants in Isolator Cages with a Variety of Contact Bedding," Contemp. Topics Lab Anim. Sci. Vo. 173, pp. 96-113 (1995) the results of studies of isolator-type cages housing mice with eight different contact beddings. The presence of ammonia and other environmental contaminants was studied.
Choi et al. in "Effect of Population Size on Humidity and Ammonia Levels in Individually Ventilated Microisolation Rodent Caging," Contemp. Topics Lab. Anim. Sci., Vol. 33, pp. 77-81 (1994), discuss the effects of population size on the buildup of ammonia and humidity in individually-ventilated microisolation cages over time as compared to static microisolation cages.
Hasenau et al. in "Microenvironments in Microisolation Cages Using BALB/C and CD-1 Mice," Contemp. Topics Lab. Anim. Sci., Vol. 32 (1) pp. 11-16 and 32 (2) pp. 58-61 (1993) report the results of studies of four different mouse caging systems for microenvironmental temperature, humidty and ammonia levels.
In Sato et al., "Dehumidification of Ventilation Air in a Barrier Maintenance System for Laboratory Animals," Lab. Anim. Sci., Vol. 39, pp. 448-450 (1989) and Wu et al., "A Forced-Air Ventilation system for Rodent Cages," Lab. Anim. Sci., Vol. 35, pp. 499-504 (1985) it was reported that ammonia is produced in greater amounts under conditions of high humidity. Desiccation was shown to be helpful in the prevention of ammonia and humidity accumulation.
Corning et al. in "A Comparison of Rodent Caging Systems Based on Microenvironmental Parameters." Lab. Anim. Sci. Vol. 40, pp. 498-508 (1991) describe two studies of four different mouse caging systems, evaluating them for microenvironmental temperature, carbon dioxide, relative humidity and ammonia levels. The cages evaluated were filter lid vs. open lid types.
Serrano reports in "Carbon Dioxide and Ammonia in Mouse Cages: Effect of Cage Covers, Population and Activity," Lab. Anim. Sci. Vol. 21, pp. 75-85 (1971) on a study of the effects of rod, wire-mesh and fibrous filter-type covers on diffusion or convection of gases produced in mouse cages. It was found that filter or mesh covers had major influences on the composition of air in the cages.
Keller et al. in "An Evaluation of Intra-Cage Ventilation in Three Animal Caging System," Lab. Anim. Sci. Vol. 39, pp. 237-242 (1989) report on a study of air distribution and air turnover rates in unoccupied shoebox mouse cages, filter-top covered cages and shoebox mouse cages housed in a flexible film isolator. They concluded that although filter-top covered cages reduce the cage-to-cage transmission of disease, the poor airflow observed within these cages could lead to a buildup of gaseous pollutants that may adversely affect the animals' health.
Tu et al. in "Determination of Air Distribution,Exchange, Velocity and Leakage in Three Individually Ventilated Rodent Caging Systems," Contemp. Topics Lab. Anim. Sci. Vol. 36, pp. 69-73 (1997) report on a study of individually ventilated rodent cages. The inefficiency of exhaust scavenging from such systems compromises their suitability for use with hazardous agents. Also, chilling and dehydration resulting from air velocity can result in animal losses due to hypothermia.
Applicant's literature review identified the following important factors in controlling the macro- and microenvironments for caged rodents:
Genetic heritage and environmentally-influenced biological responses.
Ventilation in filter top cages does not necessarily increase with increasing room ventilation air exchange rates.
Filter tops can significantly affect cage ventilation performance.
Cage conditions varied widely with cage location in a room.
Desiccation was shown to be helpful in the prevention of ammonia and humidity accumulation.
Improved cage washing procedures and animal room cleanliness may reduce the concentrations of bacteria that produce ammonia.
Bedding type can significantly affect ammonia generation.
A recent study suggests that groups of five mice display a behavioral and autonomic thermoneutral zone that is similar to individual mice, including a temperature warmer than standard housing temperatures. This suggest that groups of mice may experience cold stress under standard housing conditions. Ammonia concentration can be reduced by increasing the supply air temperature.